preparation

Cover potatoes with salted cold water
by 1 inch and bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, 15 to 20 minutes. Reserve 1/4 cup cooking water, then drain potatoes in a colander.

While potatoes are boiling, soak onion in cold water 15 minutes, then drain in a sieve and pat dry.

When potatoes are just cool enough to handle, peel and quarter, then toss with lemon juice and salt in a large bowl.

Whisk together onion, sour cream, mayonnaise, dill, and reserved cooking water in a bowl until blended. Whisk in salt and pepper to taste, then add to potatoes and toss to coat. Serve warm.